1. Field of the Invention
The present invention relates to a drive device for backlash-free conversion of rotary motion into linear motion by use of a toothed belt and of a drive gear positioned to engage the two belts.
2. Brief Description of the Background of the Invention Including Prior Art
High requirements are associated with the capacity and accuracy of measurement and tool machines where slides, supports, tools or tool tables perform linear motions and with machines with cross support slides that perform simultaneous motions in the directions of two coordinates. In these cases, the linear motions are not only to be performed rapidly, that is, with large acceleration and deceleration values, but also with highest accuracy in order to maintain desired production tolerances. Finally, the motion of these machine parts is to be free of slippage, to be free of backlash and play, and to be free of vibrations in order to allow mass production under reproducible conditions with the aid of numeric controls. The requirements lead in practical terms, because of the high loading of the component parts of the linear drive, to undesired wear and tear, which result in the long run in inaccuracies and scattering during the numeric positioning of the slides or tools. The known and usually employed drive systems for the performance of linear adjustment motion, which, for example, comprise threaded spindles, screws, recirculating ball nut and screw drives, gear racks, hydraulic or pneumatic pistons or toothed belts. These systems and devices are all applied because of their properties and, in fact, as is understandable, always where based on the requirements their favorable properties are used to especial advantage.
However, when the question arises of providing linear drive where all conditions are fulfilled to a very high degree, for example, in case of a large adjustment path under large adjustment speed, for example, 1.0 m/sec, then it is difficult to obtain a backlash-free, vibration-free and accurate positioning. In particular, this is difficult especially in continuously repetitive runs. Thus, for practical purposes, all known drive systems are eliminated despite their specific advantages because their application under the conditions described increases their disadvantages to such an extent that such linear drive for an optimum solution for the requirements is hopeless from the beginning.
Systems of this kind comprise at least one gear rack running in an adjustment direction. The two sides of the drive gear are provided with arcuate guides, which maintain the toothed belt in such engagement with the gear rack that a section accepts the load and only a short defined section of toothed belt remains out of engagement with the teeth and where furthermore the toothed belt is provided as an endless loop. Such a drive is known for example, in principle, from the German Pat. No. 2910373.
A device is taught in U.S. Pat. No. 3,824,871 where a container is placed in a back and forth motion by means of a chain drive actuated by chain gears. This is employed for a relatively uniform distribution of spread bulk materials in a storage space. Precision motion and precision positioning cannot be achieved with this known device. In particular, during a change in the direction of motion in this device, the required play between the elements engaging each other is substantially interfering.
The drive device operating with a toothed belt taught in U.S. Pat. No. 3,850,043 comprises a separate adjustment provision for eliminating the backlash of the toothed belt. The backlash between the toothed belt and the gear rack however cannot be eliminated with this additional element. It is in addition, a disadvantage of this device that depending on the length of the toothed belt, that is, depending on the position of the slide, there occur different changes in length of the toothed belt under the same force action. Thus inaccuracies in positioning result, which cannot be tolerated in case of a numerically controlled machine tool.